This proposal requests support for the development of the first clinically practical, flat-panel, solid-state, digital imager for real-time radiotherapy treatment verification. The imager will be based upon a new imaging technology being pioneered by the investigators involving thin-film transistors and sensors. The imager will have an optically sensitive area approximately 46 cm by 50 cm consisting of 1024 x 1120 pixels. This surface will be made up of four 23 cm by 25 cm self-scanning, solid state detector arrays. Each array will comprise 512x560 imaging pixels arranged in a regular two-dimensional pattern with a pixel pitch of 0.450 mm. Each pixel will consist of a hydrogenated amorphous silicon (a-Si:H) photodiode coupled to an a-Si:H field effect transistor. The imager, when used in conjunction with a metal plate/phosphor screen (or with a thick crystalline scintillator) and with a readout system to be developed in this project, will permit imaging at frame rates from 4 frames per second to 1 frame over the entire treatment dose. The imager will be very compact with a thickness approaching that of a radiographic film-cassette. It will offer high resolution, high quality images limited only by the incident detected radiation quanta thereby facilitating real-time verification of the treatment setup. Studies of the array signal properties as well as observer-independent performance variable measurements (MTF, DQE, NPS, SNR) will be performed in order to understand and optimize the performance of the imager. Studies will be performed to assess the extent to which the imager facilitates the evaluation of object displacement and to see how low the imaging dose may be set while still producing a clinically acceptable image quality. The successful completion of this project will result in a highly compact, clinically practical imager whose image quality will be better than that of existing camera-fluoroscopy imagers and, possibly, even rivaling that of film. With such attributes, the imager will provide the first example of a new generation of portal imagers which will serve as a powerful new tool in the difficult problem of radiotherapy treatment verification.